The present invention relates generally to lighting, and more particularly to a ceramic arc discharge lamp.
Discharge lamps produce light by ionizing a fill material such as a mixture of metal halides and mercury with an arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge chamber which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a xe2x80x9cdosexe2x80x9d, emits a desired spectral energy distribution in response to being excited by the electric arc. Halides generally provide spectral energy distributions that offer a broad choice of light properties, e.g., color temperatures, color renderings, and luminous efficacies.
A conventional metal halide lamp is fabricated by charging, in a light-transmitting quartz tube, mercury, an inert gas, e.g., argon (Ar), at least one kind of rare earth halide (LnH2 or LnH3: where Ln is a rare earth metal, e.g., scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu), and H is chlorine (Cl), bromine (Br), iodine (I)), mercury (Hg), and at least one kind of alkali earth halide (NAX: where NA is an alkali metal, e.g., sodium (Na), lithium (Li), cesium (Cs), potassium (K), or rubidium (Rb)) and sealing the tube.
The requirement for metal halide lamp operation at high temperature often excludes the use of quartz or quartz glass for the discharge vessel wall, and necessitates the use of a ceramic material for the discharge vessel wall. Ceramic discharge lamp chambers were developed to operate at higher temperatures, e.g., above 950xc2x0 C., for improved color temperatures, color renderings, and luminous efficacies, while significantly reducing reactions with the fill material. A ceramic discharge chamber is often made from metal oxide, such as, for example, sapphire or densely sintered polycrystalline Al2O3, as well as from metal nitride, for example AlN. Typically, ceramic discharge chambers are constructed from one or more components which are slip cast, molded, extruded or die-pressed from a ceramic powder.
Ceramic metal halide (CMH) lamps provide many benefits. For example, CMH lamps combine a high luminous efficacy with excellent color properties (among them general color rendering index Ra.xe2x89xa780 and color temperature Tc between 2600 and 4000 K) making them highly suitable for use as a light source for, inter alia, interior lighting.
In general, CMH lamps are operated on an AC voltage supply source with a frequency of 50 or 60 Hz, if operated on an electromagnetic ballast, or higher if operated on an electronic ballast. The discharge will be extinguished and subsequently be re-ignited in the lamp, upon each polarity change in the supply voltage.
Extension of CMH technology from low wattage to high wattage (for example, from less than or equal to 150 watts to a wattage greater than, for example, 250 watts) introduces several problems. Arc tubes are more prone to cracking due to the larger size. Furthermore, halide cost per volume becomes more important due to the larger volume of the arc tube legs. Similarly, it is harder to achieve Ra greater than 80 due to the lower mercury density associated with larger wattage at fixed voltage.
One mechanism for dealing with the problem associated with developing high wattage ceramic metal halide lamps is the selection of the appropriate arc discharge fill. Because of the effect on all characteristics of the lamp, including, lumen output, color temperature, efficiency, interaction with the arc discharge chamber, and starting capabilities, only to name a few, fill selection is a very complicated undertaking.
According to one aspect of the invention, a metal halide lamp having a ceramic discharge chamber is provided. The ceramic discharge chamber contains an ionizable fill. The fill is comprised of mercury and halides of at least sodium, thallium, an alkaline earth metal, and from greater than 0 to less than 15% of a rare earth element as a molar fraction of the halide fill constituents. Cesium halide may also be added to the fill to improve lamp life when the lamp is burning horizontally.
According to a further aspect of the invention, a metal halide lamp having a ceramic discharge chamber is provided. The ceramic discharge chamber contains an ionizable fill. The fill is comprised of mercury and halides of at least sodium, thallium, an alkaline earth metal, and from greater than 0 to less than 15% of three rare earth elements as a molar fraction of the halide fill constituents.
According to another aspect of the invention, a dose for a metal halide lamp is provided. The dose is comprised of mercury and halides of at least sodium, thallium, an alkaline earth metal, and from greater than 0 to less than 15% of three rare earth elements as a molar fraction of the halide fill constituents. According to a further aspect of the invention, a metal halide lamp having a ceramic discharge chamber is provided. The ceramic discharge chamber contains an ionizable fill. The fill is comprised of mercury and halides of at least sodium, cesium, thallium, an alkaline earth metal, and from greater than 0 to less than 15% of three rare earth elements as a molar fraction of the halide fill constituents.